a. Field of the Invention
The present invention relates to an acoustic coupler provided between an ultrasound probe and a surface of a body to be diagnosed.
b. Description of the Related Art
An ultrasound probe including an ultrasonic transducer is used for diagnosing an affected part in a human body by holding the probe to a surface of the human body. During holding the probe to the body, the ultrasonic transducer transmits an ultrasonic wave which is formed into an ultrasonic beam for scanning the body, and receives ultrasonic echoes from the body along the beam.
As well known, the ultrasonic transducer has two types, a single type having a single ultrasonic transducer and an array type having plural ultrasonic transducers arranged either in a line an in a matrix. Therefore, the size and shape of the ultrasound probe is varied in accordance with the type of the ultrasonic transducer.
When the surface of the body is soft so that the ultrasound probe can be tightly fitted to the surface of the body and when the affected part is located from the surface of the body as deeply as the ultrasonic echoes can be focused well with a sufficient depth of field, the probe can be used in a state being directly held to the surface of the body through a thin layer of water, vegetable, oil, glycerin or liquid paraffin. The thin layer prevents multi-reflection caused by an air gap between the probe and the surface of the body.
On the other hand, when the surface of the body is not soft or not flat to the ultrasound probe as in a case that the thyroid gland or the carotid artery is diagnosed by the ultrasound probe, it is hard to hold the probe to the surface of the throat tightly without leaving the air gap. Furthermore, when the affected part is located near the surface of the body as in a case of diagnosing the thyroid gland, carotid artery or mamma cancer, it is hard to detect ultrasonic echoes from the affected part with sharp resolution, because the echoes are focused within a thin depth of field. In these cases, an acoustic coupler having a thickness of approximately 1 to 10 cm is used between the ultrasound probe and the surface of the body. The thickness of the acoustic coupler is varied in accordance with the flatness of the surface of the body and/or the location of the affected part from the surface of the body.
The material of the acoustic coupler must have low acoustic attenuation and acoustic impedance required to be equal to the surface of the human body. This kind of material will be called an "acoustically transparent material" or simply a "transparent material" hereinafter. As the transparent material for the acoustic coupler, water put in a bag has been used, using a transparent elastic material such as silicon rubber for the bag. Water is easily obtained and can be used in a hygienic condition so there is no worry of making the body dirty. However, there were problems that the bag would have been broken, resulting in splashing water on the body. This problem has been solved by using a gelled material for the acoustic coupler. A gelled material can be made so as to be acoustically transparent. Further, it is easy to apply to the surface of the body without leaving an air gap, and it is easy to remove from the surface, leaving no fragment of the gel material. Such gelled material will be simply called a "gel" hereinafter.
In regard to the gel and its application to the acoustic coupler, there have been many patents (Japanese laid opened patents). For instance: SHO 59-11708 to IMAI on Jan. 25, 1984 discloses a gel applicable to the acoustic coupler, such as a silicon rubber gel, a water gel, an oil gel or a gel mixed with water and oil; SHO 59-49750 to MIYAMOTO et al. on Mar. 22, 1984 discloses a gel called a poly vinyl alcohol (PVA) gel which a kind of a hydrated gel containing approximate 80% of water content, having large elasticity and hardness and very small acoustic attenuation. The elasticity and the hardness depend on a refrigerating temperature and a frequency of repeating refrigeration-and-thaw of a PVA material, performed in a fabricating process of the PVA gel; SHO 59-82838 to MIYAMOTO et al. on May 14, 1984 discloses the details of the PVA gel, for instance, about composition, relation between water content and acoustic attenuation and between water content and hardness of the PVA gel, and the patent explains that the PVA gel can be plastically formed so as to be fitted to the ultrasound probe when the PVA gel is hardened properly; SHO 61-288842 by FURUTA et al. on Dec. 19, 1986 discloses a probe applicator the same as the acoustic coupler, consisting of a U shaped case adaptable to the ultrasound probe and water or some kind of gel put in the U shaped case; SHO 63-36173 by MACHIDA et al. on Feb. 16, 1988 discloses an acoustic coupler mountable on an ultrasound probe, consisting of a case having a tapered shape so that one end of the case to be fitted to the probe is large and the other end of the case to be put on the surface of the body is small and water or a colloidal solid put in the tapered case; and SHO 63-49146 by MACHIDA Mar. 1, 1988 discloses an acoustic coupler consisting of a case having different portions for mounting the ultrasound probe water put in a bag or a colloidal material put in a bag installed in the case, wherein the different mounting portions are for mounting the ultrasound probe in different directions toward the surface of the body. This is caused by the fact that the probe is often required to be oriented to the surface of the body in at least two directions, a perpendicular direction to the surface of the body and a parallel direction to the surface of the body, the latter being required in such a case that blood in a blood vessel running near the surface parallel to the surface of the body is to be diagnosed by ultrasonic waves.
Some of the above-mentioned patents disclose acoustic couplers each including a case having a membrane, to which the surface of the body is applied, and a gel or a colloidal material filled in a gap between the case and the membrane. However, in this case, there is a problem that the ultrasonic waves (the ultrasonic echoes) are attenuated by the membrane. The attenuation is caused by an acoustic attenuation of the membrane of course, mainly caused by multi-reflection due to the different acoustic impedance between the body and the membrane.
Generally, a gel is a material which is difficult to be mechanically caught or held by a small supporting area because of its lack of hardness and viscosity. Therefore, the gel is often used in a state of being simply laid on the surface of the body without putting the membrane between the surface and the gel, and the ultrasound probe is applied on the laid gel. However, this is inconvenient for one (an operator) operating the ultrasound probe, because the operator always must contact the gel with both hands independently of treating the ultrasound probe. As a result, the gel has been used inevitably by putting it in a case having a membrane even though multi-reflection occurs due to the membrane.
However, in the above-mentioned, SHO 59-49750 and SHO 59-82838 by MIYAMOT et al. teach, respectively about the acoustic coupler made of the PVA gel. In particular, SHO 59-82838 teaches a concept that the PVA gel can be fitted to the ultrasound probe by hardening the PVA gel properly. This is important because SHO 59-82838 suggests that the PVA gel can be used as an acoustic coupler when directly combined with the ultrasound probe without using the case and the membrane. However, SHO 59-82838 teaches nothing about the structure of an acoustic coupler using the hardened PVA gel.
Irrespective of a general gel or a PVA gel, an acoustically transparent gelled material is difficult to be applied to an acoustic coupler unless a case and a membrane are used in the acoustic coupler. Since, as stated before, the gelled material has small hardness and viscosity, it has been a difficult problem to overcome.